My task one day had been finding out about the difference between a cesspit and a septic tank. The simplest answer seems to be that a cesspit is closed (it is just a big tank in the ground) but a septic tank is open (everything goes in, solids are trapped, liquids come out again). Both have to be drained.
Here is a photo of the land near our septic tank in Portelas.
Everything around it is the colour of straw, for the simple reason that it is straw. The house was empty for years and the grass grew to waist height. One of the local worthies got an industrial mower onto 3 neighbouring plots, including ours, and baled the crop as winter feed for GNR horses. The straw is what was left. It has not rained in many months so the grass is not re-growing.
The green circle of green grass in the photo is, I assume, where the fluid is allowed to drain from the tank. The grass looks healthy enough, despite the fact that the fluid contains liberal doses of bleach and clothes washing liquid. Maybe the large number of showers is diluting it enough to stop this from being a problem.
To empty the tank of solid waste, there should normally be a manhole cover, with vehicular access for a waste lorry. We appear to have neither of those. At the moment, it appears our tank was installed by a neighbour, but the work has not been completed. It requires a further €6,000 to install a pump. Presumably the pump is needed to pump solid waste from the tank to a mains sewage system, removing the need for a manhole cover and vehicular access.
But the norm would be a manhole cover, used for removal of solid waste.
What does this have to do with Madeleine McCann? As far as I can see, it reduces the likelihood of body disposal via a septic tank or cesspit. I know there was at least one case in the UK in which the body of a murdered woman was put into a cesspit. The police failed to find anything during a search of the property, until a neighbour told them there was a cesspit under the garage. The corpse had not dissolved, and presumably, the cesspit had not been drained since her death.
Whilst the cesspit/septic tank route would be available to a local with knowledge of such and the ability to lift up a manhole cover, it is not a viable body disposal or body concealment option for the McCanns or other members of the Tapas 9.